Symmetry breaking in the relaxed S-1 excited state of bianthryl derivatives in weakly polar solvents

The flash-photolysis time-resolved microwave conductivity technique (FP-TRM C) has been used to investigate the nature of the relaxed S-1 state of 9,9' -bianthryl (AA), 10-cyano-9,9'-bianthryl (CAA), and 10,10'-dicyano-9,9'-bia nthryl (CAAC). Changes in both the-real, Delta epsilon' (dielectric constan t), and imaginary, Delta epsilon " (dielectric loss), components of the com plex permittivity have been measured. The dielectric loss transients conclu sively demonstrate the dipolar nature of S1 for all three compounds in the pseudopolar solvents benzene and 1,4-dioxane, and-even in the nonpolar solv ents n-hexane and cyclohexane. The required symmetry breaking is considered to result from density and structural fluctuations,in the Solvent environm ent. The dipole relaxation times for AA (CAAC) are ca. 2 ps for the:alkanes and 7.9 (5.3) and 14 (14) ps for benzene and dioxane, respectively. The ti me scale of dipole relaxation for the symmetrical compounds is much shorter than that for rotational diffusion and is attributed to intramolecular, fl ip-flop dipole reversal via a neutral excitonic state. The dipole moment of the transient dipolar state is estimated to be ca. 8 D, that is much lower than the value of ca..20 D determined- from the:solvatochromic shifts in t he fluorescence in intermediate to highly polar solvents which corresponds to close to complete charge separation. For the asymmetric compound, CAA, a dipole moment close to 20 D is found in all solvents, including n-hexane. Dipole relaxation in this case occurs on a time scale of several hundred:pi coseconds and is controlled mainly by diffusional rotation of the molecules . The mechanism and kinetics of formation of the dipolar excited states are discussed in the light of these results.